INS Arihant (ATV) News and Discussion -2

[Boreas]

The other point is who tells the truth about max depth reachable and max speeds?

Australians

[Kersi D]

The other point is who tells the truth about max depth reachable and max speeds?

Australians

Possible. For their Collins the max speed is 0.84365 knots and max depth reachable is 6526.0983 mm

K

[shiv]

Just a few thoughts - with no real information. A submerged speed of 8 knots translates to about 15 kmph or 360 km in 24 hours. Not great for going places - the sub would have to travel a week to find itself in foreign waters 2000 km away - but it would be virtually undetected.

Even twice that speed (16 knots) is hardly a sprint. Sailboats of yore could probably better 16 knots.

It strikes me if I pull out high school physics from the recesses of my mind that boats can be speeded up if you reduce their area of contact with water by cheating - as in hydrofoil boats, or by sheer raw power. I suspect (with no proof) that at speeds below 30 knots a ship that weighs 5000 tons and a sub that weighs 5000 tons would have a roughly similar surface area in contact with water. So if the ship can do 25 kts on the surface it should be able to do the same underwater given the same amount of power.

So I wonder if low underwater speeds have everything to do with stealth. Apart from engine/gear noise - the ship will surely set off turbulence that is detectable. Also I am guessing that no matter how high tech and stealthy you make the propeller/screw the faster you turn it the more likely it is to be noisy. That means that stealthy subs cannot turn on full power in one go and "burn rubber" as it were by letting the screw go whirrrrrrrrrr "Avast! Full speed ahead!!" as the ship accelerates. Even the acceleration must be slow to avoid turbulence and noise. That means the propeller is gradually accelerated from whoooooosh-whoooooosh to whooosh-whooosh-whooosh to whoosh-whoosh-whoosh-whoosh.

[ragupta]

If India goes for type 216/U-216 sub, it could incorporate a NR in a local version of it.
I guess this is why the second line is delayed.

http://www.navyrecognition.com/index.ph ... iew&id=264

[Singha]

well if a torpedo is coming at you, stealth is moot - you need to escape and accelerate as fast as possible.

the layer of water in contact with the props actually 'boils as steam' iirc under pressure of the blades...maybe the 7 blade thing turns slower and minimizes this

[vina]

Even twice that speed (16 knots) is hardly a sprint. Sailboats of yore could probably better 16 knots.

Most modern cargo ships cruise at around 15 knots around most economical speed, a trade off between fuel consumption and number of trips you can made. This is a function of fuel price. Before 1970, ships traveled at 22 knots + , after fuel crisis , speeds dropped to 15 knots. Today, it is only "high speed" ferries and some container ships that go to 22knots and beyond

It strikes me if I pull out high school physics from the recesses of my mind that boats can be speeded up if you reduce their area of contact with water by cheating - as in hydrofoil boats, or by sheer raw power. I suspect (with no proof) that at speeds below 30 knots a ship that weighs 5000 tons and a sub that weighs 5000 tons would have a roughly similar surface area in contact with water. So if the ship can do 25 kts on the surface it should be able to do the same underwater given the same amount of power.

Some basic YinJin Ear Ring here. There are two components to a ship's resistance. They are 1) The wave making resistance, you make waves, you lose energy. Waves are made because of the air water boundary on the surface.. this is measured by the Froude Number (google for it) and 2) The skin friction resistance, which you alluded to, that is a function of the Reynold's Number (google for it).

Coming to ships and submarines. Now unlike a surface ship , a submarine has NO , repeat NO, Zilch, Nada, Zero, Cipher, Shoonya wave making resistance, because it is fully submerged and there is no air/water boundary to make waves.

So, your point that a ship with a given wetted surface area is going at 25 knots, a sub with the exact wetted surface area has to go at the same speed is not true. The sub in fact will be going substantially faster (assuming that both the ship and sub are hydro dynamically efficient and "ship shape" and have similar powering).

Google for the United States "Guppy" program and why it came about and why modern submarines have a "tear drop " hull shape, compared to WW-II subs which have a shape like a "surface boat", with the top sides fully closed.

So I wonder if low underwater speeds have everything to do with stealth. Apart from engine/gear noise - the ship will surely set off turbulence that is detectable. Also I am guessing that no matter how high tech and stealthy you make the propeller/screw the faster you turn it the more likely it is to be noisy.

There is a tactical speed beyond which your sensors will become "blind" with your own noise (though modern signal processing probably obviates some of this.. think Bose active noise cancelling headsets) and passive sensors will get lost in all the noise and you might have to start using active sonar . And yes, true about the propeller/screw, but with good and careful design, you can make sure that it is well manageable within the design speeds.

That means that stealthy subs cannot turn on full power in one go and "burn rubber" as it were by letting the screw go whirrrrrrrrrr "Avast! Full speed ahead!!" as the ship accelerates. Even the acceleration must be slow to avoid turbulence and noise. That means the propeller is gradually accelerated from whoooooosh-whoooooosh to whooosh-whooosh-whooosh to whoosh-whoosh-whoosh-whoosh.

There is such great inertia in the system and ship machinery, that in any case, it is not a "quantized jump" from say 200 rpm to 500 rpm like in a motor boat with an outboard engine in a juffy. More than the prop, usually in ships/subs much of the source of vibration is not the props per se, but other rotating machinery, in fact the drive shafts (long rods, prone to vibration) are notorious, and hence efforts to isolate all machinery on mountings so that they cant transmit vibrations and other stuff. Subs with the motor in the aft and small short shafts and single screw will have mercifully less of such a problem. Put the engine amidships and long drive shafts driving props in the rear like most surface combatants and older WWII types...yeah, big problem.

In fact, even flushing to toilet in a submarine has to be extremely quiet. If you take the "parryware" toilet or even the TFTA "American Standard" or Uber TFTA Japanese "TOTO" and install it on a sub and if a sailor takes a dump and flushes, the "woosh" can be picked up if not isolated.

[Singha]

the bulbous whale nose shape is the most efficient underwater shape for a large vessel....whales have evolved into it and some of them swim 100+km a day. the swordfish & tuna family (tuna, swordfish, sailfish, marlin) can swim faster but obviously internal volume is not ideal.

the shape of a modern N-boat is almost like a killer whale.

[vina]

the layer of water in contact with the props actually 'boils as steam' iirc under pressure of the blades...maybe the 7 blade thing turns slower and minimizes this

How that "cavitation" works is like this. As the prop spins, the local pressure drops and the dissolved air in the water trends to escape by forming bubbles (aka boiling, just like water boils at lower temperatures at altitude it takes literally forever to cook rice/daal in hills and Bangalore compared to the plains as well, unless you use a pressure cooker).

What can be done is to make the props spin slower (ie bigger props, spinning slower, far more efficient), but then you run into practical limits on how big you can given the draft /clearance etc and you cant have the props get exposed when running on surface (props hitting water after going through air.slap..slapp is sure fire way of getting the thing to pieces by shock under high loading) and also increase the "Area Ratio" (ie amount of surface area of the prop in actuality, compared to what it would be if it were a solid disc.. less area more efficient, due to less loss due to surface friction.. more area, (accomplished by increasing number of blades and or increaseing the area per blade, so that a greater part of the full disc is blade and not empty) less loaded prop (ie thrust per area)and hence local pressures remain elevated preventing bubbles from forming) . One more trick that is used on surface ships is to be okay with the bubbles forming, but make sure that the bubbles collapse a bit downstream of the blades so that they are not damaged, but that cant work in a sub , where you really want to prevent any cavitation.

[Indranil]

Shivji ... your analysis is not true.

The coefficient of friction between air and the ship is very different (much lower) than the ship and the air. So much so that there is an entire branch of hydrodynamics which deals with how to handle the layer of ship where the transition happens from water to air.

The design of marine vessels remains more of an art than a science in large part because dynamic similitude is especially difficult to attain for a vessel that is partially submerged: a ship is affected by wind forces in the air above it, by hydrodynamic forces within the water under it, and especially by wave motions at the interface between the water and the air.

Also water being a much more denser fluid has a much higher drag. In lay mans terms think of this if you are trying to push a sub through fluid, then you have to displace the fluid in front of it. The heavier the fluid, the more effort you have to make. For vacuum it will be zero, for air it will be higher, for water it will be even higher, even higher through quicksand and so on.

Hence any sub at full power will be much faster when at the surface than when completely submerged.

P.S. Did not see Vina's answers ... he has explained things in detail.

[Austin]

Hence any sub at full power will be much faster when at the surface than when completely submerged.

True for WW2 era design sub that was more of boat shape and spent more time above water then under.

But the more modern design of Tear Drop Hull( Kilo ) or Albacore type ( Virginia ) are much faster under water then when surfaced.

[Indranil]

^^^ I stand corrected.

[Anujan]

Most of the energy is wasted in cavitation around the screw and by generating the bow wave and wakes. Tear drop-shaped hulls generate huge waves and wide wakes and lose a lot of energy when surfaced. This is not a problem when submerged. Also beyond a particular depth, the higher pressure of the water causes the screw to be more efficient by reducing cavitation and improving the linear flow of water.

Both these cause increased propulsion energy to be utilized to move the sub forward rather than waste it by creating turbulence in the water.

[vina]

linear flow of water

If you mean "laminar" flow, well, foggedabout it. Other than the front part of the bow (I distinctly remember a very nice pic of a US N boat, Virginia class? running on the surface with such flow clearly visible) the rest of the flow will be partially or fully turbulent, more so towards the rear as the flow is decelerating and tends to separate out.

Great way to identify laminar flow btw. If it is "transparent" and you can see through the water, it has to be laminar..sort of like looking through a glass tank, coz the layers aren't mixing.

[Cain Marko]

Jumping from all this technical discussion to something more strategic, if the babus are serious about poking around in the S.China Sea, a 2nd Akula would be of great use, not to mention them backfires. Otherwise, best stay put and dhoti shiver in waters closer to home!

[shiv]

well if a torpedo is coming at you, stealth is moot - you need to escape and accelerate as fast as possible.

Modern torpedoes travel beteeen 50 and 100 knots so no sub would seriously be able to outrun a torpedo coming at it except under exceptional circumstances - provided the sub has sensors to detect the incoming torpedo. Anti- torp weapons may be an idea but I don't know of any

[merlin]

Every sub would have sensor to detect an incoming torpedo - a passive sonar. Something traveling at 60+ knots should be noisy and detectable no?

[uddu]

Most torpedoes are said to have speeds of 45-55 knots. The closer one is to a torpedo, the lesser the chance of escaping. If lucky and at a distance then the best way is to run away from it at maximum speed. This will give more time to release lots of countermeasures with the only hope of the torpedo selecting the countermeasure instead of the sub.

[Austin]

Merlin most torpedoes today have variable speed pump jet propulsion , so they can attain silent slow speed or top sprint speed during terminal attack , generally they are guided by fire optic cable and can attack the target approaching the target from different direction/speed making it difficult to get to its source which is the submarine stretching across many km , the onboard sonar on the torpedo ( and/or submarine sonar ) will give all the needed information on the target and the operator can control it till its very close and then cut loose the FO/Wire and let the Torpedo attack at full speed either passively or using topredo own active sonar , giving the target very little time to react , you just need a small hole on the sub pressure hull , the depth and pressure will do the rest.

A good example is British Spearfish HWT
http://baesystems.com/ProductsServices/ ... 91715.html

Once launched, Spearfish has an inherent capability to operate autonomously. Using a wire command and control link, Spearfish can exchange information with the launch submarine at long distances to provide and receive continuous situation updates.

Powered by a high efficiency gas turbine engine driving a ducted pump jet propulsor, Spearfish is capable of variable speeds. Ultra quiet at low speed ensuring maximum discretion and optimum passive search capability, its high power density enables Spearfish to attain exceptional sprint speed in the terminal stage of an attack.

Highly effective against all known and predicted submarine and surface targets, including countermeasures, the operational software allows tactical flexibility, whilst its built-in stretch potential permits through-life upgrading to counter changing threats.

Capable of prosecuting attacks from long range, Spearfish perfectly complements the powerful sonar and combat control capability of the modern submarine.

[uddu]

These seems to be the basics of Torpedo evasion for a sub
http://www.scribd.com/doc/49609458/101/Torpedo-Evasion

[merlin]

Ok so sounds a bit more difficult to detect a torpedo if its running ultra quiet. Wonder if it would be more easy to detect the launching sub and fire an anti-sub missile/rocket to force it to cut of the torpedo and evade the incoming.

Also torpedo, once its guidance cable is cut, should be amenable to full jamming via noisemakers right?

[uddu]

No. It's more like chaff and flares used by aircraft when chased by a missile. Something similar but here it takes place in slow motion. The chances of the torpedo choosing the sub over the countermeasure exits. and the number of torpedoes that can be fired against a sub is much higher.
Each and every platform varies in it's torpedo evading game. Subs can dive deep, can run silent. Surface platforms don't have that luxury but they do have capabilities like helicopters that help to detect subs using active sonar techniques. Subs always prefer to be in silent mode or passive sonar mode listening to whatever is in their surrounding. Once a surface ship detects a contact, it will use rockets to target that area. This may be followed by Torpedo launch followed by launch of a helicopter to detect that sub. Once spotted by a surface combatant and if the sub is not attacking, then the chances of its survival is almost nil. The only thing the sub can do is go so slow and keep diving to the maximum depth and move away from the possible searching area of the surface combatant. Almost impossible.

[uddu]

The ultra quiet torps are the new inventions. It makes evading torpedoes much more difficult. So subs must try to get undetected in the first place (Nowadays it's much more important than earlier times, earlier there was a second chance. Today it may not be there).

[uddu]

http://www.bharat-rakshak.com/MONITOR/I ... oseph.html
Austin, is that written by you? Needs update on India's latest torpedoes Varunastra, LWT training torpedoes etc.
http://livefist.blogspot.com/2011/05/in ... ready.html Is there something called Takshak Torpedo?
India's torpedoes
http://livefist.blogspot.com/2011/01/in ... -2011.html

[Austin]

Yes its almost been a decade , its times to rewrite considering there is so much of development happening around , may be after DexExpo we might see more information on Indian Torpedo development coming out and would re-write it.

I remember reading a long time back we were working with Kazak on Wake Homer Torpedo but nothing after that , hopefully we have worked and quietly inducted it.

[Singha]

the kazakh collab is the NPOL HWT for ships I think. not sure if same can be put in subs unmodified.

[Christopher Sidor]

Point to ponder. What will be the range of the nuclear missiles that will be carried by the Indian akula? And with those ranges, what will be the utility of the sub ?

[Austin]

Ok so sounds a bit more difficult to detect a torpedo if its running ultra quiet. Wonder if it would be more easy to detect the launching sub and fire an anti-sub missile/rocket to force it to cut of the torpedo and evade the incoming.

Difficult question to answer as there is so much of tactics and training that comes into play in Submarine and ASW game and most of it remains a tightly classified subject , but common trade magazine knowledge would tell you detecting a submarine in a environment favorable to submarine will be like finding a needle in haystack and you might end up devoting a large resource of surface ships and ASW to detect and track it.

I think if a sub captain is ready to fire a torpedo at a target he might fire more than one at a target , his confidence level must be very high that he can get the target and get out as weapons are limited on submarine and there is always this chance that a weapons fire might expose him and he would not take unwanted risk against his crew and sub

Also torpedo, once its guidance cable is cut, should be amenable to full jamming via noisemakers right?

If the engagement is short range in range of couple of km then modern torpedoes are very much autonomous and very intelligent as well and can deal with different jammers and you might end up dealing with multiple torpedoes coming for different direction with some intelligent algorithim and fuses to deal with different type of target.

[nachiket]

Point to ponder. What will be the range of the nuclear missiles that will be carried by the Indian akula? And with those ranges, what will be the utility of the sub ?

Zero. The Akula will not carry any N-missiles because it doesn't have any vertical missile launch tubes like the Arihant. The only missiles the Akula can fire are those which can be launched from the torpedo tubes, like the SS-N-15 Starfish. I don't know if the Nerpa can fire the Klub. The SS-N-15 can have a nuclear payload but India does not have any SS-N-15s. The Nerpa lacks the 650mm torpedo tubes of the older (non-modified) Akula class. Those subs could fire the SS-N-16 too. I don't think the Nerpa can.

Besides, the Akula is a purebred hunter-killer. Why would you want it to fire nuclear missiles?

[Anurag]

Point to ponder. What will be the range of the nuclear missiles that will be carried by the Indian akula? And with those ranges, what will be the utility of the sub ?

Zero. The Akula will not carry any N-missiles because it doesn't have any vertical missile launch tubes like the Arihant. The only missiles the Akula can fire are those which can be launched from the torpedo tubes, like the SS-N-15 Starfish. I don't know if the Nerpa can fire the Klub. The SS-N-15 can have a nuclear payload but India does not have any SS-N-15s. The Nerpa lacks the 650mm torpedo tubes of the older (non-modified) Akula class. Those subs could fire the SS-N-16 too. I don't think the Nerpa can.

India can mate these missile with a N-warhead, but I'm guessing under some transfer treaty India cannot use this sub to launch Nukes (just my guess!)

[Ravi Karumanchiri]

^^^ Rakshaks,

Forgive me for saying so; but some of this discussion is badly out of date.

As technology advances, constraints are opened-up, new limits are pushed for, tactics change and therefore also strategy.

For example, the VA-111 Shkval super-cavitating, rocket-powered torpedo, capable of travelling in excess of 400kmph underwater!

I'll bet this torpedo doesn't come with a stealth-mode, but when it's doing better than 400 kmph, and it's self-guiding, everything changes, and for example, the relevance of many of the above posts becomes..............

............. well, I'd rather not say.

BTW: There is mention that the Shvaal is in the Indian arsenal, and has been for many, many years. Also, a lot has been made about an aggresive export sales business and that the weapon can rely solely on intertial guidance (ring-laser gyros?) and on-board terminal guidance not reliant on IR or radar (mmw?, magneto?), so no wires, and because it is highly maneuverable and extremely fast, and can be fired in any direction against targets reportedly up to 60 km away (!), it is very definitely changing the "traditional" calculus in submarine warfare (and ASW). (Strong rumour: Russia has it operationalized and probably a few "specials" (nuked); China has a downgraded version and has been unable to buy more or reverse-engineer it (the Russians claim its key tech cannot be reverse-engineered, so they don't mind selling it far-and-wide); France has a few; India too (and there has been a case made for India to have some "specials" of this type too, capable of quietly stalking a CBG at slow-speed, then accelerating for the kill at better than 400(!) kmph and letting-off a big one. I can well imagine, since it is a rocket capable of flying underwater, that such a weapon could easily breach the surface to deliver a nuke above the surface, if surface effects are deemed more effective (which I would imagine is true).

Suggested reading:

From 2003: http://www.deagel.com/Torpedoes/Shkval_a002340001.aspx
From Canada: http://www.cbc.ca/fifth/main_armstrade.html
Good overview: http://www.globalsecurity.org/military/ ... shkval.htm
Some blog: http://india97780.yuku.com/topic/868/Aw ... w3RdIGwV7c
Interesting: http://www.indiadefence.com/navaldoct.htm

Do some more Googling, there is plenty out there on this particular weapon.

[Philip]

Media reports that the second ATV will be launched by the year end/early 2013 and that the third sub is also making good progress too.

[Nikhil T]

Second nuclear submarine headed for year-end launch

At a time when diminishing operational availability of its conventional submarine fleet has put the Navy in dire straits, it has some reason to cheer.

Informed sources told The Hindu that the construction of a second Arihant-class nuclear submarine, to be named INS Aridaman, is moving fast at the Shipbuilding Centre (SBC) in Visakhapatnam. It is slated for launch by this year-end or in the first quarter of next year.

“The boat, under outfitting now, is headed for a year-end launch. Meanwhile, hull fabrication is on for the third Arihant-class nuclear-powered ballistic missile submarine,” the sources said. “Unlike surface vessels, submarines are fully outfitted before launch, which makes it a prerequisite for its weapons to be tested and ready well in advance.”

The first submarine of the class, INS Arihant, launched in July 2009, has just completed its harbour acceptance trials and is set to undergo the crucial sea acceptance trials in February.

“This will be followed by weapon trials before the submarine is formally inducted into the Navy, hopefully in 2013, when the country will attain the much-desired nuclear triad,” the sources said. Concurrently, nuclear-powered submarine INS Chakra, borrowed on a 10-year lease from Russia mainly for training purposes, will be inducted in the latter half of 2012.

Troubled by the eroding strength of its conventional underwater arm, the Navy's ‘blue water' aspirations remained in the realm of wishful thinking, with the force failing to add even a single submarine to its inventory in the last decade.

With the Advanced Technology Vessel (ATV) programme to indigenously design and build nuclear-powered attack submarines gaining momentum after years of indecision and disorientation in the 1990s, the goal, claimed the sources, was within reach now.

Chief of the Naval Staff Admiral Nirmal Verma said last year that once commissioned, INS Arihant would be deployed on ‘deterrent (combat) patrol.'

Although it would be home-ported in Visakhapatnam, the submarine, armed with nuclear-tipped K-15 or B-5 ballistic missiles and having a range of about 750 km, would offer effective deterrence against Pakistan, the sources pointed out.

The missiles are developed under the Sagarika programme.

Displacing about 6,000 tonnes, the 112 metre-long Arihant-class of boomer submarines are powered by indigenously-built 80-MW nuclear power plants. Each submarine is said to store 12 K-15 missiles besides torpedoes and torpedo-launched cruise missiles.

OBSOLETE FLEET

While the ATV project is on track, the Navy finds its back against the wall having to operate a flagging fleet of Russian Kilo-class and German HDW conventional diesel-electric submarines, 14 in all, 75 per cent of which are over the hill.

“The decline in the operational availability of submarines [as low as 40 per cent] has seriously compromised the force's vital sea denial capability. The absence of Air Independent Propulsion, which obviates the need for conventional submarines to surface frequently for recharging their batteries thereby enhancing their endurance is another debilitating factor,” said the sources.

[Rahul M]

Ravi ji, it's not a foregone conclusion like that. as always the devil lies in the details. like most revolutionary tech, the first iteration has some problems which would require significant maturing of the associated tech to be a viable weapon system which can replace conventional torpedos.

firstly, the range is less than 10 km, you can imagine the risk to the launching vessel in trying to close the distance between it and the target. most likely the target would destroy it before it got a chance to fire the shkval.
secondly, it still has problems with homing. last I heard the torpedo would be fired to a fixed location and it could not accept realtime changes in targeting.
shkval was after all built as a suicide weapon against large targets like carriers.

right now the shkval is like a soldier's handgun, used only when the enemy is far too close to use rifles. it can't replace rifles nor does it make rifles redundant.

[Singha]

I have heard the skhval would be n-tipped and be fired in general direction of a approaching torpedo, its goal being to destroy the torpedo / make the enemy sub cut guidance wire and run way from the shock wave.

[keshavchandra]

Last night I had a dream that India has developed an advanced ATP for its coming submarine fleet and the shocking part was Pakistan requested for TOP and final instalment on its ageing sub fleet.
seems funny ....

[Pratyush]

I have heard the skhval would be n-tipped and be fired in general direction of a approaching torpedo, its goal being to destroy the torpedo / make the enemy sub cut guidance wire and run way from the shock wave.

Nuke launch authority is best not delegated by any state.

[Singha]

but this is a kitty kitty nice kitty 'defensive' weapon. in any case it would be used if fired upon as a last resort. or used on a CBG if a war had started. the big Granit's off the Oscar's surely had a subset that was n-tipped for when the s*** really hit the fan.

[Pratyush]

Agreed, but those days are long gone. Them bad old days.

[prahaar]

In the above article about ATV, it says

Displacing about 6,000 tonnes, the 112 metre-long Arihant-class of boomer submarines are powered by indigenously-built 80-MW nuclear power plants. Each submarine is said to store 12 K-15 missiles besides torpedoes and torpedo-launched cruise missiles.

How is that ATV has a length of 112 meters but a displacement of 6000 tonnes whereas Akula-II has a length of 113m but a displacement of about 8500 tonnes (12500 tonnes submerged)? Also, weird part is that Akula-II is a SSN which is supposed to be more sleek than a SSBN. Is it easier to make long small diameter subs than short large diameter subs (just like in case of missiles Agni-II vs Agni-III)? Of course, it is possible that the real dimensions/tonnage/etc of Arihant is classified. It might also point towards easy "bloating" for additional silos after initial proof of concept in Arihant.

[Singha]

the Akula/Tyhoon/Oscar are all double hull subs. that increases their tonnage and makes them look quite fat.

the Arihant like most other subs is single hull. a good comparison with with 688I sub which is 110m, 6100 tons empty and 6900t submerged. in Akula the diff between empty and submerged is a clear 3500t which indicates it has a massive reserve buoyancy and can take some battle damage in terms of flooded chambers vs single hull designs. downside is it needs a more powerful reactor => more reactor grade uranium.

the definitions of SSN has been getting bigger the virginia is 7500t, seawolf 9000t and the yasen is as big. the prolific 688 subs are smaller and the british and french even more so. similarly FFGs have taken over the DDG space and the 7000t horizon class poses as a frigate.

wiki:
The Akula incorporates a double hull system composed of an inner pressure hull and an outer "light" hull. This allows more freedom in the design of the exterior hull shape, resulting submarine with more reserve buoyancy compared to its western analogs. This design requires more power than single-hull submarines[citation needed] because of the greater wetted surface area, which increases drag.